How to politely say in a paper that most of the published research on a topic is by the same person?

First off the fact that one group is dominating will be implicitly understood if you provide a paragraph or two summarizing their work and cite their publications. In such a paragraph you could add words like "ground-breaking" or phrases that describe the dominance of the group on the field.

You could for example start the paragraph by saying:

"The work concerning risk assessment of [...] has largely been carried out by [the research group] (example citations)."

(I hope you see the main idea of my attempt) Then you follow up on what they have done and cite their work.

It is just important to keep the tone as neutral as possible and let the citeable work speak for itself. Spending one or more paragraphs describing the work should be enough to make everyone understand the message.

1. Most of the available work on this risk factor in this subgroup comes from papers that all share one author. He is the leader regarding this question

   This you can freely express in your paper: it is a statement of fact, and if you provide evidence to back it up (papers, reviews, book chapters, etc.), there is no reason for it to be interpreted as a hostile statement. It is actually quite interesting.

2. He also holds a patent for assessment of this risk factor. […] This guy has published virtually everything there is that reports this positive association

   Again, it's a statement of fact, not a problem. You can cite a patent if you want, and stating that in a neutral light is easy.

3. and possesses shares of a company that develops and plans to sell diagnostic tests to assess this risk factor. […] and he would profit enormously if it were true.

   This, on the other hand, I would be very wary of writing that in a review. First, because it is not a scientific statement, so its place in a scientific paper is not clearly defined. Secondly, because it could (and probably will) be interpreted as having a strongly negative implication, possibly to the extent of suggesting very serious ethical issues.